Why can't Meta (or any other shady company/ organization) do that now anyway. Just set up an innocent looking server, populate it with a small number of accounts to make it look legitimate, federate and start sucking in data. Do you really think every single federated server is run by people with hearts of gold and pure intentions? Your shit is already getting harvested, there's no stopping that. They don't need Threads if all they want is to index posts.
Meta sucks, I get it, but I think a lot of the fear Threads is generating is way overblown.